1. Field of the Invention
The present invention relates to a heat-radiation type liquid-level sensor output of which indicates the present weight of the liquid in a tank.
2. Prior Art
The resistance of an electrical resistor increases with an increase in temperature as shown in FIG. 6 and a change in resistance is proportional to the temperature of the electrical resistor. For example, when a current from a constant current source Io flows through a liquid level measuring resistor Ro such as an Ni wire of a predetermined length as shown in FIG. 7B, the resistor generates an amount of heat to increase the resistance of the resistor Ro. If this heat-generating resistor Ro is substantially dipped in liquid, the resistance thereof will decrease. This is because the thermal resistance between the resistor and the liquid surrounding the resistor when the resistor is placed in liquid is smaller than that between the resistor and the gas surrounding the resistor when the resistor is placed in air. The overall resistance of Ro will be smaller with increasing length of the resistor Ro dipped in the liquid as shown in FIG. 7A; therefore the voltage drop across the resistor Ro will decrease. This indicates that detecting the voltage variation across the resistor Ro enables the measurement of the level of the liquid.
The voltage V.sub.(x) across the resistor Ro is given as follows: ##EQU1## where
L: total length of the resistor Ro,
X: length of the resistor Ro dipped in the liquid,
Kb: non-dimensional quantity which is a function of, for example, a thermal resistance, and causes a change in surface temperature of the resistor Ro relative to that of the air with which the resistor contacts when the current flows through the resistor,
Ro': resistance per unit length of the resistor Ro,
Ka: non-dimensional quantity which is a function of, for example, a thermal resistance that causes a change in surface temperature of the resistor relative to that of the liquid with which the resistor contacts when the current flows through the resistor, and there is a relation of 1&gt;Kb&gt;Ka.
FIG. 4 shows one conventional temperature-compensated heat radiation type liquid level sensor. In FIG. 4, the resistor Ro is a liquid level measuring resistor made of, for example, a length of nickel wire and Rt is a temperature compensating resistor, made of an electrically conductive wire similar to the Ro, and Rf is a feedback resistance having a temperature coefficient negligible compared to those of the resistors Ro, Rt. Rt does not generate a significant amount of heat compared to the resistor Ro because little current flows through the temperature compensating resistor Rt.
One end of the liquid level measuring resistor Ro' is grounded and the other end thereof is connected with a constant current source Io so that a constant current I flows through the resistor Ro.
The voltage across the resistor Ro is fed through a buffer amplifier A which has a unity gain and serves to isolate the resistor Ro from a succeeding circuit. The output of the buffer amplifier A is supplied to an inverting feedback amplifier which is constructed of an operational amplifier Op, the temperature compensating resistor i.e., input resistance Rt, and the feedback resistor Rf. The gain of the feedback amplifier is given by Rf/Rt, therefore the gain will decrease with temperature if Rt increases with temperature. The change in resistance of the resistor Ro appears as a voltage change across a point a and the ground in accordance with the change in liquid level, and is supplied to the inverting feedback amplifier through the buffer amplifier A. The voltage is then amplified by the inverting feedback amplifier to be outputted as a voltage indicative of the liquid level.
The resistor Rt is disposed in a liquid tank such that it experiences the same ambient temperature as the resistor Ro does. Since the resistor Rt does not generate the detectable amount of heat, the resistance thereof is not affected by the change in liquid level but is dictated by only the ambient temperature, i.e., liquid temperature and air temperature. When the voltage across the resistor Ro increases with the ambient temperature, the input voltage to the inverting feedback amplifier also increases. At this time, the resistance of the resistor Rt also increases and therefore the gain of the inverting feedback amplifier decreases. As a result, the output of the inverting feedback amplifier does not change. In this manner, the error due to the change in resistance of the measuring resistor Ro is compensated by the change in resistance of the compensating resistor Rt.
The voltage at a point b is given by the following equation.
Selecting Rf=Rt and selecting the temperature coefficient of Rt equal to that of .alpha..sub.0 of Ro, ##EQU2##
.DELTA.T: a change in ambient temperature.
.alpha..sub.0 : temperature coefficient of the resistor Ro, Rt and is given; ##EQU3## where
Ra: the resistance at a temperature Ta,
Rb: the resistance at a temperature Tb.
The thermal resistance from resistor to air of the resistor Ro is greater than the thermal resistance from resistor to liquid of the same resistor Ro. Thus it can be said that 1&gt;Kb&gt;Ka.
The above-described prior art level liquid sensor is of a type in which the liquid level measuring resistor Ro in the form of, for example, a length of wire is supported at both ends thereof by a supporting member so that the resistor wire is held taut. Therefore, when handling the liquid measuring resistor in the form of wire or ribbon, care must be taken so that the resistor is not damaged or cut off. In addition, the effect of liquid level change due to thermal expansion thereof is not compensated.